Modem fabrication processes offer the microwave circuit designer a wide range of techniques and materials for realizing passive planar components, such as bandpass filters. This paper investigates the effects that the fabrication process has on the performance of microwave planar circuits, so as to provide the circuit designer with useful practical information leading to the optimum choice of materials and processes. In order to compare the technologies, single-layer and multi-layer band-pass filter circuits were fabricated and tested at 25GHz and 2.5GHz, respectively.